Expressing his views on drug policy, Attorney General Jeff Sessions said marijuana legalization wouldn't be "good for us." He also doubted reports of marijuana's effectiveness fighting opioid addiction, adding "we need to crack down more on heroin." (Reuters)

Speaking this morning before the National Association of Attorneys General, U.S. Attorney General Jeff Sessions expressed doubt that marijuana could help mitigate the opioid abuse epidemic.

“I see a line in The Washington Post today [link added] that I remember from the '80s,” Sessions said. "'Marijuana is a cure for opiate abuse.' Give me a break. This is the kind of argument that's been made out there to just — almost a desperate attempt to defend the harmlessness of marijuana or even its benefits. I doubt that's true. Maybe science will prove I'm wrong.”

The stakes are pretty high here. After all, opioids killed 33,000 people in 2015, up from around 8,000 in 1999. As the head of the Department of Justice, Attorney General Sessions oversees the Drug Enforcement Administration, which just last year reaffirmed its belief that marijuana has no medical value and hence should remain illegal (which makes it substantially more difficult for researchers to conduct studies).

Here's a run-down of where the evidence on marijuana and opiates stands.

Marijuana is great at treating chronic pain.

This is the big finding, and the one from which all the others spring. Surveying the entire known universe of studies about the medical efficacies of cannabis, the National Academies of Science, Medicine and Engineering found “strong evidence” showing marijuana is effective at dealing with chronic pain in adults, relative to a placebo. The National Academies study is the most thorough review of the literature on marijuana to date, conducted by some of the nation's leading substance use researchers.

There’s been a lot of research done into this question in recent years. Here’s what it says:

States with medical marijuana laws see fewer opiate deaths.

According to a 2014 study in JAMA Internal Medicine, states with medical marijuana laws between 1999 and 2010 saw, on average, about 25 percent fewer opiate overdose deaths than states without such laws. What's more, the effect of a medical marijuana law appeared to grow over time — more lives were saved each additional year after the laws' implementation, suggesting an effect from more people taking advantage of the programs.

This, of course, is just an observational study. It looks at the correlation between medical marijuana uptake and opiate deaths, but it isn't able to say that the former definitively caused the decline in the latter. But these findings were soon borne out by other research.

Access to medical marijuana dispensaries is associated with less prescription painkiller abuse, and fewer overdose deaths.

This study was somewhat more rigorous than the previous paper, drawing on a larger data set that stood up to greater statistical scrutiny. As one of the authors told me at the time, “If this is for real, it means that there are other ways that are less dangerous [than prescription painkillers] for people to deal with chronic pain.”

It found that the typical physician in a medical marijuana state prescribed 1,826 fewer painkiller doses for Medicare patients in a given year. Why? Some seniors don't need painkillers if they have access to medical pot (related: seniors and people in late middle age are among the fastest growing marijuana use demographics).

Among chronic pain patients, marijuana use is associated with less opiate use.

A study published last year in the Journal of Pain found chronic pain patients who reported marijuana use were 64 percent less likely to report opiate use, more likely to report good quality of life, and less likely to report negative side effects from their medication.

“This study suggests that many chronic pain patients are essentially substituting medical cannabis for opioids and other medications for chronic pain treatment, and finding the benefit and side effect profile of cannabis to be greater than these other classes of medications,” the authors conclude. “More research is needed to validate this finding.”

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“More research is needed.” Researchers universally say they need to do more studies to tease out the nature of the relationships seen in the studies above. In particular, what's sorely lacking are controlled experiments that allow doctors to treat either chronic pain or opiate dependency with medical marijuana.

But as of right now that research is incredibly difficult to do because of how marijuana is classified by the federal government. As a Schedule 1 controlled substance, the government has deemed that marijuana has “no medically accepted use.”

Researchers can still work with it, but it requires jumping through an onerous system of legal hoops that can sometimes take years to navigate. It took nearly a decade for one researcher trying to use marijuana to treat PTSD in military veterans to obtain all the approvals necessary.

The scientific evidence available so far indicates marijuana holds great promise for mitigating the effects of the opiate epidemic. Researchers desperately want to know more, but as long as the Drug Enforcement Administration considers marijuana a Schedule 1 controlled substance, they'll have a hard time getting more answers.